The column labelled 'U' is a count of undefended Pawns in a position and the column labelled 'W' is a count of Pawns that are defended only by the King. The column labelled 'U+W' is, as you've undoubtedly guessed, the sum of 'U' and 'W'. The last column 'Ct' shows the number of chess960 positions with that specific number of weak Pawns. For example, the fourth row says there are 190 positions with both a single undefended Pawn and a single Pawn protected only by the King.
The sum of the last column shows that, of the 960 positions, there are 842 with at least one weak Pawn. The most interesting positions are those with a high number of 'U+W'. In my previous post on 'Undefended Pawns', I already listed the four positions with three undefended Pawns, but the two positions with four weak Pawns are something new.
As you would expect, the two positions with four weak Pawns are twins: SP115 BNQRNKRB and SP835 BRKNRQNB. In SP115 the a- & h-Pawns are undefended, while the e- & f-Pawns are protected only by the King. This is quite extraordinary and it would definitely be worth seeing how master level players have coped with these two SPs in practice.